A teenager suffered life-threatening injuries when the car he was driving came off the road and hit a garden wall while being followed by police.
The Citroen AX driven by the 17-year-old ended up on its roof following the crash on the A377 Lymington Road in New Milton, Hampshire.
The front-seat passenger, also 17, suffered a broken arm in the incident at around 0135 BST on Saturday.
A Police Complaints Authority investigation has been launched.
A spokeswoman for Hampshire Constabulary said that officers in a parked police patrol car saw the Citroen go past and decided to follow.
Moments later, they came across the vehicle on its roof, resting against a lamp post.
It is not yet known why the driver, who has not yet been identified but lives locally, lost control of the car.
Police are appealing for any witnesses to come forward.